1. Field of the Invention
The present invention relates to a liquid crystal display panel having an optically compensated bend (OCB) mode.
2. Description of the Related Art
Liquid crystal display panels have widely been used as image displays for computers, automobile navigation systems, monitors and TVs. TN and STN modes using nematic liquid crystals are popular as liquid crystal display modes for the liquid crystal display panels. Other liquid crystal display modes using ferroelectric liquid crystals and the like are also known and provide a higher response speed and a wider viewing angle. These display modes, however, require further improvement in impact resistance and thermal properties. In contrast, as an optically compensated liquid crystal display mode that provides a higher response speed and a wider viewing angle, there is an OCB mode in which liquid crystal molecules are aligned in parallel without being twisted. This OCB mode is focused on use in video equipment and active development thereof has been carried out.
In general, an active matrix liquid crystal display panel has a structure in which a liquid crystal layer is held between an array substrate and a counter substrate. The array substrate includes a plurality of pixel electrodes arrayed substantially in a matrix, a plurality of scanning electrodes disposed along rows of the pixel electrode, a plurality of data signal electrodes disposed along columns of the pixel electrodes, and a plurality of switching elements disposed near intersections between the scanning electrodes and the data signal electrodes, for example. Each of the switching elements is formed, for example, of a thin-film transistor (TFT), and turns on to apply the potential of one data signal electrode to one pixel electrode when one scanning electrode is driven. On the counter substrate, a counter electrode is disposed to face the plurality of pixel electrodes arrayed on the array substrate. A pair of pixel and counter electrodes forms one pixel together with a pixel area of the liquid crystal layer, and the alignment state of liquid crystal molecules within the pixel area is controlled by the electric field corresponding to the drive voltage held between the pixel electrode and the counter electrode.
In an OCB mode liquid crystal display panel, the liquid crystal molecules are in a splay alignment before supply of power. This splay alignment is a state where the liquid crystal molecules are laid down, and obtained by alignment films which are disposed on the pixel electrode and the counter electrode and rubbed in parallel with each other. A display operation of the liquid crystal display panel begins after an initialization process for applying a transfer voltage to the liquid crystal layer upon supply of power to transfer the alignment state of the liquid crystal molecules from the splay alignment to a bend alignment by a relatively strong electric field corresponding to the transfer voltage. The alignment state of the liquid crystal molecules is maintained in the bend alignment during the display operation, so as to attain the higher response speed and wider viewing angle peculiar to the OCB mode.
In addition, the alignment state of the liquid crystal molecules is inverse-transferred from the bend alignment to the splay alignment when a long period has elapsed in a no-voltage-applied state or a voltage-applied state where a voltage is applied but this voltage is below a level at which the energy of the splay alignment and the energy of the bend alignment are balanced. In the OCB mode liquid crystal display panel, black insertion driving is employed as a driving system for preventing the inverse transfer. In black insertion driving, an inverse-transfer preventing voltage and a display voltage corresponding to a video signal are alternately applied to the liquid crystal layer as the drive voltage at every frame cycle to maintain the bend alignment. Since the OCB mode liquid crystal display panel is a display panel of a normally white mode, the inverse-transfer preventing voltage corresponds to a voltage for a black display. Thus, this driving scheme is called black insertion driving. Further, the ratio of a period of applying the inverse-transfer preventing voltage to the total period of applying the display voltage and the inverse-transfer preventing voltage is called black insertion ratio.
In manufacture of the liquid crystal display panel, as shown in FIG. 7A, after an array substrate 1 and a counter substrate 2 are individually formed, rubbing treatment is applied to the alignment films on the array substrate 1 and the counter substrate 2. Thereafter, the array substrate 1 and the counter substrate 2 are bonded by use of a sealing resin layer 3. The sealing resin layer 3 is applied to surround a liquid crystal filling space and form an open part left as an inlet 4. Driver circuit elements are disposed along a first side of the array substrate 1, and the inlet 4 is disposed near a second side of the array substrate 1 that is opposed to the first side. The rubbing treatment for each alignment film is carried out in the same direction from the second side to the first side so as to avoid electrostatic destruction of the driver circuit elements. In FIG. 7A, RD1 denotes the rubbing direction of the alignment film on the array substrate 1, and RD2 denotes the rubbing direction of the alignment film on the counter substrate 2. A liquid crystal material is applied from the inlet 4 to the liquid crystal filling space as the liquid crystal layer LQ, and the inlet 4 is sealed with a sealing member 5.
It is inevitable that impurity ions get into the liquid crystal layer LQ during the above-mentioned manufacturing process. Specifically, the sealing member 5 is the main impurity ion source that supplies a significant quantity of impurity ions to the liquid crystal layer LQ. Such impurity ions decrease the insulation resistance of the liquid crystal. Thus, the display characteristics are impaired due to decreased voltage retention. Further, upon application of the drive voltage, the impurity ions are moved in the liquid crystal layer LQ. When the impurity ions are distributed unevenly, an image fault such as non-uniformity in display occurs. For example, Jpn. Pat. Appln. KOKAI Publication No. 9-54325 discloses a technique of providing an ion trap electrode in addition to electrodes arranged in one direction on a substrate and sequentially applying a high potential pulse to the electrodes to prevent uneven distribution of the impurity ions. However, it is difficult to use this technique as a solution to the problem occurring in the OCB mode liquid crystal display panel.
In the OCB mode liquid crystal display panel, liquid crystal molecules within the liquid crystal layer LQ are set in a bend alignment to perform a display operation. The orientation angle of each liquid crystal molecule significantly changes between a white display state in which a small voltage is applied to the liquid crystal layer LQ and a black display state in which a large voltage is applied to the liquid crystal layer LQ. The change in the orientation angle causes drifting of the liquid crystal molecules. Thus, a flow of the liquid crystal molecules in the drift direction occurs in the liquid crystal layer LQ. The direction of this flow coincides with the rubbing directions of the alignment films for aligning the liquid crystal molecules in the OCB mode liquid crystal display panel. When impurity ions are moved by the flow in the rubbing directions RD1 and RD2 shown in FIG. 7A, the impurity ions DF having high concentration in the vicinity of the inlet 4 are diffused in the liquid crystal layer LQ as shown in FIG. 7B. Further, non-uniformity in display occurs because the electric charge retention is locally decreased as a result of continuous black insertion driving. This can be confirmed, for example, by an operation of continuously displaying an image of a test pattern shown in FIG. 7C in the form of black insertion driving and then displaying an image of a whole black pattern. In this case, the later image is not displayed entirely in black, and gray stripes shown in FIG. 7D are observed as burn-in portions. As shown in FIG. 7C, the impurity ions drift from the white display regions and concentrated in the black display regions. The gray stripes are created in portions where the electric charge or applied voltage retention is locally decreased by the concentrated impurity ions.
It is difficult to solve the burn-in problem occurring in the OCB mode liquid crystal display panel by applying a high potential pulse in the same manner as the above-mentioned technique.